It's possible he is simply molting. Odd time of year for but possible. Check the areas with feather loss - does he have new pin feathers developing? If there are no pin feathers - are there any other turkeys or birds who could be damaging the feathers?
Maybe he is rubbing his tail on something and dragging his belly on the ground a bit? He looks like a Broad Breasted White turkey and they frequently have weight related issues and often drag their bellies on the ground a bit which causes them to lose feathers.
He could just be molting though, as @QueenMisha suggested.